The present invention relates to a mixing head assembly, and in particular to a mixing head assembly for mixing polyol and isocyanate to form high quality polyurethane foam.
Polyol and isocyanate are mixed to form polyurethane foam. The polyurethane foam is then placed into a mold to form polyurethane objects. Polyurethane foams can be used in a number of industries for a number of purposes. For example, rigid polyurethane foams are used in the auto and other industries for structural reinforcement, preventing corrosion and damping sound and vibration. Rigid polyurethane foams are also used as refrigeration and cooler insulation. Furthermore, flexible polyurethane foams can be used in seating cushions, arm rests and in other similar applications.
Heretofore, polyurethane foam producing methods, including structural reaction injection molding and reinforced reaction injection molding, have been used to produce flexible or rigid foam. Polyurethane foam has been made using low pressure foaming processes and high pressure foaming processes. In a low pressure foaming process, after reactive components are introduced at a low pressure into a mixing chamber through a nozzle, the components are mixed by high-speed rotation of an impeller, and are introduced into a molding. However, due to economical and environmental factors such as cleaning, mixture loss, inferior working conditions, bad quality or the like, the low pressure foaming process has typically been replaced with the high pressure foaming process. Meanwhile, the high pressure foaming process typically includes injecting reactive components at a high pressure by a nozzle to mix the reactive components by an impingement force. Thereafter, the mixture is introduced into the molding apparatus to form an object. The system for performing the high pressure foaming process typically comprises a tank for polyol, a tank for isocyanate, pumps for pumping the polyol and isocyanate, a mixing head assembly, a driving hydraulic unit, and an electronic control unit. The mixing head assembly introduces and mixes the reactive components, which is a major part for determining the quality of the final foam product.
Typical mixing head assemblies inject the reactive components in a horizontal or vertical direction. However, several methods have been proposed for improving the mixing efficiency and laminar flow at a discharging port by providing a swirl inducing chamber or an additional device. However, since the typical mixing head has a complicated construction, there are some drawbacks, such as difficulty of the maintenance, expensive manufacturing price and shortening of life cycle.
FIGS. 5–7 illustrate a typical mixing head assembly 10. The mixing head assembly 10 includes a mixing chamber 12. The polyol and isocyanate are injected into the mixing chamber 12 through a first nozzle 16 and a second nozzle 18. In use, the typical mixing head assembly 10 is connected to a hydraulic tank and to polyol and isocyanate feed lines (not shown). When a pump of the hydraulic tank is operated, a cleaning piston rod 20 is retracted from a discharge pipe 14 of the mixing chamber 12 and a mixing chamber piston rod 22 is retracted from the mixing chamber 12. Concurrently, the first nozzle 16 and the second nozzle 18 are opened to inject polyol and isocyanate at an initial high pressure of 120 to 200 bar into the mixing chamber 12.
Since the polyol and isocyanate are injected into the mixing chamber 12 in the form of opposing and impinging jets through the first nozzle 16 and the second nozzle 18, the polyol and isocyanate impinge primarily on each other. Furthermore, the first nozzle 16 and the second nozzle 18 could have axes that are neither co-linear nor co-planar. Accordingly, in this situation, the typical mixing head assembly 10 is considered to be a “dual-tilt” mixing head. The dual-tilt mixing head forces the jets of polyol and isocyanate to impinge secondarily on a wall of the mixing chamber 12, thereby forming a first swirl due to the impinging energy. The mixture consisting of the polyol and the isocyanate is passed from the mix chamber 12 to the discharge pipe 14 to form a second swirl. The mixture is passed from the mix chamber 12 to the larger diameter discharge pipe 14 to form a laminar flow, and then is introduced into the mold. After a predetermined amount of the mixture is introduced into the mold, the mixing chamber piston rod 22 extends to move the mixture out of mix chamber 12 and into the discharge pipe 14. Thereafter, the cleaning piston rod 20 is extended to move the mixture out of the discharge pipe 14 and into the mold, thereby completing the pouring process of the two mixed components from the mixing head assembly 10 into the mold.
Accordingly, a mixing head assembly solving the aforementioned disadvantages and providing an improved mixing of the reactive components is desired.